Train v. Natural Resources Defense Council, 421 U.S. 60 (1975), is a United States Supreme Court case in which the Court held that the EPA must approve a State Implementation Plan if it meets the criteria under the Clean Air Act.

The EPA Administrator was required to approve implementation plans meeting the statutory criteria and to promulgate substitutes for those found deficient.

A number of states included variance procedures in their implementation plans, which the EPA approved.

In an opinion delivered by Justice Rehnquist, the Court concluded that the Fifth Circuit erred in holding that postponements were the exclusive means by which individual sources could obtain relief from state implementation plans.

[2] The Court further held that as long as the "ultimate effect of a State's choice of emission limitations is compliance with the national standards for ambient air," a state is "at liberty to adopt whatever mix of emission limitations it deems best suited to its particular situation.